Currently available gravity roller conveyors typically use a pair of structural members to support each row of load-bearing rollers. Such a structure necessitates tedious fabrication techniques to assure uniform spacing between all of the structural members throughout the length of the conveyor. It is often desirable in such systems to stagger the placement of successive rollers to reduce the gap over which a load is unsupported. Typically, staggering is achieved merely by adding one or more additional rows of rollers with their centers longitudinally offset from one another. Recent conveyor designs have incorporated rollers mounted on alternate sides between pairs of support structures; however, the need to accommodate the axles of successive rollers has restricted the ability of such designs to minimize the gap between successive rollers. To date, the only conveyor systems providing minimal gaps between successive rollers require an excessive number of structural supporting members. Such designs exacerbate the fabrication difficulties of maintaining a proper spacing between the structural supporting members.
To avoid the use of ballbearing type journals between the axles of conveyor rollers and the structural supporting members, some designs have used specially machined configurations of axles for retentively engaging apertures in the support members with various types of flanges, bushings and retainers. Other designs have used sleeve-type ballbearing assemblies interposed between the axles of the rollers and specially formed receptacles in the supporting structural members. Unique axles and specially formed receptacles both incur additional tooling and fabrication costs. Moreover, the often intricate interrelation between the specially formed receptacles and the unique axles or bearings tends to hinder the repair or replacement of damaged rollers, thereby unnecessarily contributing to the maintenance cost of such conveyor systems.
Often rollers used for conveyors are filled with deformable tires of various configurations and materials. A major difficulty with currently available conveyors using deformable tire conveyors (both gravity conveyors using hysteresis speed control tires and push-line conveyors using minimum hysteresis tires) however, arises from a tendency of the tires, particularly tires made of elastomeric materials, to develop a compression set when a cargo load is allowed to rest at one place. Compression set causes a flat spot on the periphery of tire, thereby causing subsequent resistance to rolling conveyance of a cargo load.
The choice of materials available in efforts to overcome the problem of compression set and rolling resistance is limited to materials exhibiting a low compression set, preferably less than fifteen percent (15%) when tested in accordance with ASTM method B for twenty-two hours at a temperature of 70 degrees Celcius. Unfortunately, this limitation on materials eliminates many deformable materials and elastomers exhibiting properties of desired hardness at required temperatures which would otherwise make those elastomers ideal. For example, urethane compounds having low compression sets have not been found to be suitable at temperature in ranges between 30 degrees F. to -20 degrees F., a temperature range invariably encountered by conveyors installed in freezers, because such materials tend to become too hard within such temperature ranges. Conversely, elastomers which remain soft enough (e.g., softer than 90 Shore A) tend to develop excessive compression set.
Another problem encountered by currently available conveyors using deformable tires in conveyors arises from the tendency of wood slats of pallets to compress when a cargo load is allowed to remain stationary, particularly where elastomeric tires extend above a narrow, rigid rim of a roller wheel. Often, enough indentation of the rim into the wood of a pallet's slat occurs to prevent free movement of the pallet. Moreover, high humidity seems to exacerbate this problem. Furthermore, softer woods are more susceptible to indentation and subsequent rolling resistance after having remained stationary while supported by currently available conveyors using rollers fitted with elastomeric tires.